Invisible wall barriers. That's the best way to do it for a province like this. Modders can remove the barriers and make extra land mods. I'm sure people, years down the line, will be making province projects for High Rock, Cyrodiil and Morrowind landmass mods. Just like how TESIII has Project Cyrodiil and Skyrim: Home of the Nords. And I think there is a Black Marsh project too.

Modding is pretty irrelevant here. Like 90% of the people who played Oblivion played it on consoles. Would they have to "suffer" from an invisible wall just to make it easier for a few mods?
In Oblivion, there has been only one landscape mod that is relevant here, since it has been released: Elsweyr.

Also, there's nothing stopping modders to simply alter the landscape if they want to make natural borders passable.
Moreover, it's easier for modders to have their landmasses in seperate worldspaces. I hear that having it in the same worldspace can cause a lot of troubles.
Finally, you have to seperate the "real" game from the modded game. These are two entirely different things.

Invisible wall would be fine..... But natural barriers more realistic